1. Field of the Invention
The present invention relates to a method and apparatus for producing high purity water. More particularly, this invention relates to a method and apparatus for eliminating contaminants in water by contacting the water with a variety of purifying media for predetermined periods of time.
2. Summary of the Prior Art
High purity or ultrapure water is necessary for a variety of reasons in commercial and residential applications. The standard method for measurement of ultrapure water is water having a specific resistance measured in megohms above a specified value. An 18.3 megohm resistance is the highest purity achievable with ion exchange resins presently available. In general, high purity water has a resistance of 10 megohms while a resistance of 18 megohms is required in the electronics industry. Another method for measuring water purity is by the determination of the total dissolved solids (TDS) present in the water. These dissolved solids are generally present as ions in the water. The quantity, total dissolved solids, is expressed as parts per million. Various analytical processes require high purity water, as the contaminants in conventional water interfere with accurate analysis. Hospitals also require water having purity levels between 1 megohm and 18 megohms. The purity of the water necessary to meet hospital requirements varies with the particular application. For instance, hemodialysis and various blood workups require 18 megohm water.
In addition to commercial applications, there has been a marked increase in the demand for high-purity drinking water among private residential customers. Individuals desire water free from the contaminants and additives generally found in drinking water derived from municipal sources. It is felt that these additives and contaminants impart unpleasant tastes to the water and detract from its overall quality. Many consider that the additives and contaminants present in municipal water are detrimental to an individual's general health.
Various methods have been proposed for obtaining high purity water. These methods include distillation, filtration or reverse osmosis. These methods generally require costly equipment, maintenance and extensive energy outlays. Distillation requires large amounts of electrical energy but fails to produce sufficient quantities of high purity water. Where filtration is used, large amounts of these expensive materials are required to achieve the desired purity levels. The filter media used tend to plug easily, requiring frequent replacement. Reverse osmosis requires extremely precise water conditions and constant monitoring to work effectively. Additionally, the osmotic media are extremely fragile and produce too little water to be cost effective in general applications. Thus, the costs of employing such methods are prohibitive for the individual or small commercial user. Such users are forced to rely on bottled purified water. Bottled water is expensive and does not ensure the user with a steady source of pure water on demand.
Additionally, most purification methods and media presently employed are contaminant-specific. A specific method or purification system is effective for the removal of only a certain class of contaminants, i.e., specific organics, inorganic salts or the like. If removal of a variety of contaminants is desired, several different purification methods or media must be employed. Such multi-contaminant purification systems are cumbersome and costly to maintain. Additionally, premature failure of one purifying method or media requires costly replacement, usually by a skilled technician, if production of high purity water is to be maintained.
Where water purification is performed, ion exchange resins are a commonly used water purification media. Ion exchange resins are designated as either cationic exchange resins, anionic exchange resins, or mixtures thereof. Ion exchange resins are capable of producing hydrogen or hydroxide ions which interact with ionic contaminants found in the water; binding them to the resin and rendering them innocuous.
Ion exchange resins of this type maintain a capacity to remove ionic contaminants, expressed as kilograms of removal (kgr) per cubic foot of the resin used. Thus, the capacity of the resin is the total amount of water which can be purified before the resin is "processed to exhaustion" i.e., the amount of total dissolved solids of the outgoing water after passage through the resin, equals that of the incoming water.
These ion exchange resins are generally used in confined cylindrical, fiberglass tanks. These vessels vary in dimensions from about 6 inches by 18 inches to about 36 inches by 72 inches; diameter to height, respectively. In tank sizes ranging from 6".times.18", to 10".times.54" (most commonly used in exchange systems) the tanks are fitted with distributor assembly. The assemblies consist of an inlet and outlet, located perpendicular to one another along the longitudinal plane. A fitting at the side allows water into the tank. The fitting at the side allows water into the tank. The fitting at the top contains a tube (also called a standpipe) extending to the bottom of the tank. Water is forced through the bed to the bottom of the tank, then through the standpipe located in the center of the tank.
The effectiveness of ion exchange resins is dependent on the contact of each unit of water with each successive resin bead. As each unit of water comes in contact with each bead, an incremented amount of contaminant is removed. Thus, the more contact between water and resin permitted, the more contaminant removed.
The present design of cylinders containing ion exchange resin permits only limited contact between water and resin beads. Because water will follow a path of least resistance through the center of the cylinder, the resin at the sides of the cylinder is only minimally exposed and contacted. This greatly reduces efficiency of the resin and the total purification cylinder. Additionally, the configurations of the various present purification devices do not provide means for effectively combining various purification methods in series.
Thus, it is desirable to provide a method and apparatus for purifying water which increases the effectiveness of the purification media used. It is also desirable to provide a method and apparatus from which high purity water can be obtained which requires a reduced amount of purifying media without sacrificing the total purifying capacity of the device. It is also desirable to provide a method and apparatus for purifying water which is economical, requires minimum space, and is easily serviced and replaced when the purification media is exhausted. It is also desirable that the method and device be suitable for combining a variety of purification media in admixture or in discrete segments as required.